The present invention relates to the authentication of printed documents and, in particular, to a method for distinguishing original printed documents from photocopied or laser-printed duplicates of printed documents.
The large variety of different types of paper documents used in commercial, governmental, and private transactions can be broadly classified into two categories. In the first category are original documents produced by offset printing, handwriting, typewriting, ink jet printers, dot matrix or other impact printers, thermal and thermal transfer printers, pen-based plotters, and other such methods. The second category of paper documents includes duplicates or copies of original documents produced by photocopy machines or laser printers.
Photocopying and laser printing technologies have greatly improved in quality and capabilities in recent years. High-resolution color printers are, for example, of sufficient quality to be used by paper currency counterfeiters. Because monetary values and legal rights are frequently associated with original documents, but not with copies or duplicates of those documents, it is very important for those holding or receiving transactional documents to be able to distinguish original documents from copies of original documents. However, because of technological advances in photocopying and laser printing, it is becoming increasingly difficult for these holders and receivers of documents to distinguish original documents from copies.
FIG. 1 introduces an example of the problem of distinguishing an original document from a copy that will be used below in the Detailed Description of the Invention. FIG. 1 shows an original bill of sale 102 and a photocopy of the original bill of sale 104 produced by a high quality, high-resolution color printer. To the naked eye, there is no perceptible difference between the original bill of sale 102 and the copy 104.
There are many reasons why a holder or receiver of the bill of sale might wish to distinguish the original bill of sale from a copy. A holder of a bill of sale might, for example, be able to present the bill of sale to the seller in order to receive a rebate, a discount on a subsequent purchase, or a refund for damaged or faulty goods. It is important for a seller or merchant to be able to distinguish the original bill of sale from a copy to avoid paying out multiple refunds, discounts, or rebates for a single purchase. The holder of a bill of sale may wish to obtain some further document or authentication based on the original bill of sale. For example, the purchaser of a car may wish to obtain title to a car by presenting an original bill of sale to a government office. Presenting a copy or duplicate bill of sale may possibly constitute fraud. The holder of the bill of sale may wish to authenticate the bill of sale to avoid possibly committing fraud, and the receiving government office may wish to authenticate the bill of sale in order to avoid licensing the car to someone other than the owner.
Besides authentication, there are other reasons for which it may be useful to distinguish laser-printed or photocopied documents from original documents. For example, a laser printer or photocopy machine might send documents that have been previously laser-printed or photocopied through a different paper path than an original document in order to avoid jamming. A need has therefore been recognized for a method for faithfully and automatically distinguishing an original document from a photocopied or laser-printed document.
The present invention provides a method for distinguishing laser-printed or photocopied documents from original documents provided by offset printing, handwriting, typewriting, and other such methods. In one embodiment of the present invention, a document is scanned with an optical scanner at low-resolution and again at high-resolution. Optical scanning provides a two-dimensional matrix, or grid, in which each cell or position corresponds to a physical position on the surface of the document and each cell contains one of two values that indicate whether ink or toner was detected by the scanner at that position on the document. Those sections of the high-resolution matrix that correspond to blank or white space in the low-resolution matrix are then analyzed to detect small, randomly-spaced microdots that appear only at high-resolution. These small, randomly-spaced microdots are present on photocopied or laser-printed documents, but not on original documents. Those documents that exhibit small, randomly-spaced microdots are thus identified as photocopied, or laser-printed documents.